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Chapter 5
OPPORTUNITIES IN PARTICULAR TECHNOLOGIES
SUMMARY
This chapter describes opportunities for research and development
where advances in electrochemical devices and processes will probably
have a significant economic impact in the near term (less than
10 years). Both new and traditional industries are considered. The
current status and needs for research and technology development, along
with some institutional issues, are examined for
Batteries and fuel cells: Technical requirements are
documented for advanced applications in ground-based vehicles, space and
central electric utility systems, communication systems, medical
applications, and weapons; associated research and development topics
are summarized.
· Biomedical science and health care: Electrochemical processes
characteristic of living systems are reviewed, including such aspects as
applications based on neuroscience, enzyme biocatalysis, adhesion and
cell fusion, and electrophoresis.
· Coatings and films: Most paints and coatings degrade by a
photoelectrochemical mechanism. Applications are summarized that
include protective coatings for automobiles, encapsulants for
microelectronic devices, electrocatalysts, and microencapsulation
techniques for controlled release of electroactive components.
· Electrochemical corrosion: A framework of opportunities is
presented with respect to corrosion research and engineering,
dissemination of information, and new control technology to reduce
corrosion losses.
.
· Electrochemical surface processing: Research and development
underlying new monolithic and composite materials, coatings,
electroplating and etching, and microelectronic devices, among others,
are highlighted.
· Manufacturing and waste utilization: Current applications and
emerging technologies are reviewed, and dominant economic considerations
are noted for electrolytic processes, electro-organic synthesis,
41
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42
coproduction of metals and anodic products, and specific applications
such as vehicles, electric power, and waste utilization.
Membranes: Directions are outlined to achieve greater
membrane stability and molecular transport and in turn to permit wider
use of energy-efficient and economically attractive membrane technology
in biotechnology, health care, and chemical synthesis.
· Microelectronics: Electrochemical phenomena are essential in
the manufacture of electronic and photonic systems as well as
responsible for the quality and reliability of such systems.
Applications and research are outlined in areas that include manufacture
of microcircuits, interconnecting networks, lightwave communication
devices, parallel processors, content-addressable memories, and
nerve-electronic interfaces.
· Sensors: Key technical problems involve materials and
fabrication methods for both gas-and liquid sensors; opportunities for
utilizing advanced microelectronics and membrane technologies are
suggested for applications in 'environmental, industrial, and clinical
systems, including in vivo monitoring of drug delivery systems.
Electrochemical science and engineering is moving extremely rapidly
in areas of advanced energy conversion devices, microelectronics, and
sensors. These technologies have significant market growth potential,
and international competition is keen. Greater support from both
federal and industrial sources would have a major impact in these areas.
BATTERIES AND FUEL CELLS
The current and emerging applications for batteries and fuel cells
are numerous and highly varied (1-4~. These chemical sources of
electrical energy are absolutely essential for life in today's world. A
sampling of current applications includes portable electric power for a
wide range of civilian, industrial, military, and aerospace applications
such as flashlights, radios, tools, medical devices (heart pacemakers, drug
delivery systems), weapons, communication equipment, alarms, signals, and
satellite power in space. All of the world's telephones operate with
batteries as standby power sources. Standby power, emergency power, and
uninterruptable power are provided by batteries for high-priority
systems such as hospitals computers, and military weapons installa
~ .
lions. Motive power is provided by batteries for hundreds of thousands
of specialty vehicles such as forklift trucks, personnel carriers, air-
port utility vehicles, submarines, torpedoes, and drone aircraft. New
battery systems are being developed with far greater specific power and
specific energy than realized in conventional batteries (Figure 5-1~.
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200
100
~0
SPECIFIC 60
POWER
(W/kg)
40
30
20
Current
_~__ Projected LiAl/FeS . Ii`,Si/FoS2 Chevette (1.4 liter)
~'A
Zn/ NiOOH · ~ ~
~ ~ rVa/S
_ ~`~- ~ ~/ Acceleration (Peak)
~ ------- `---r'y---------------
%~\ \~t
-- - --^\---`\ ~\ 1
~ ~ ~ 1
/` ~ ~
Pb/PbO2 \ ~ ~
~ 1 1
. ... ~ .
10 . .
t --i; Uman {AV~ )
, . ..
.. . ..
I I I I I
l I I
l! . 11
&., . , .. 1 1 1 1 .. 1
10 20 30 40 60 80100 200 300400 600 8001000
SPECIFIC ENERGY (W.h/kg)
FIGURE 5-1 Specific energy versus specific power for several batteries
under development, compared to the Pb-PbO2 battery. Note that
high specific power and high specific energy are offered by some of the
new batteries.
Emerging applications for fuel cells and batteries are oriented
toward higher performance and longer life. In the near term (within a
decade), advanced electrochemical power sources will be available to act
as the principal motive power source on a commercial basis for delivery
vans, buses, and other fleet vehicles. In the far term (more than a
decade away) these types of power sources will become available for
higher-performance automobiles, rail vehicles, high-performance
submarines, ships, and perhaps aircraft. Stationary energy storage
applications include storage in electric utility networks (near-term
availability), wind-powered electric systems (near-term), and
solar-electric systems (far-term). Fuel cells (Figures 5-2 and 5-3) are
strong candidates in the far term for high-efficiency (greater than
50 percent) commercial electric utility power generation and stand-alone
power generation for shopping centers, hospitals, military
installations, and industry, as well as for remote power generation in
developing countries. Also, fuel-cell-powered vehicles of all types are
a far-term possibility. At present there is an increased interest in
ultrahigh-performance electrochemical systems for defense and space
applications. Some of these systems could find application within the
next decade. New high-performance miniature batteries are in demand for
medical applications, including mobile heart-pump systems, drug-delivery
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44
H2
H2O
- 3-
_ , ~ = =: ~
an-' - =
==
~9
. -=W
---~ L
~ =-=:
.. ~ _ ~
~ ~ ~-
Air
I Electrolyte l
H2 Anode
2 2 2
O2 Cathode
H2O
N2, H2O
FIGURE 5-2 Schematic cross section of a hydrogen-oxygen fuel cell, the
heart of fuel cell systems. Such systems may be a major power source
for electric utilities and electric vehicles.
systems, and electrically powered prosthetic devices of various types.
A number of these medical applications will be fulfilled within the next
several years.
The performance capabilities required of batteries and fuel cells
vary according to the type of application. Some sample requirements are
given in Tables 5-1 and 5-2.
There are a number of barriers to achieving the requirements for
various battery and fuel-cell applications. In general, improvements
are needed in the areas of initial cost, lifetime, and performance.
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45
1orooo
3000
2000
u'
a)
U1 1000
~ BOG
. _
~ 600
cn
, 400
o
a)
200
100 1 1
20
.
;
~40V
\~\ 3.0 · Lit
2.0 \\\
ZniAir~Li/~\
Li4Si/FeS2t Li/Fe \\\
\/Ai e:Na/S~ <~Na/SbC13
Li4si/Fcse~LiAllFes2\ \ \
LiAI/F~ 6~
In/NiOOH \\\ \
\ \ Nags \ \
Cd/Ag2O2- ·Fe/NiOOH \ \ \
Zn/HgO \ \ \ \ \
\ ·Cd/NiOOH \ \
1 1 1
-
-
~\\~
1 ~ 1 \1 ~ N~ 1360
100
Equivalent Weight' g/equiv
104:
a)
llJ
C'
._
· _
Cal
a)
. En
O
1o3 ~
1
1 000
FIGURE 5-3 Theoretical specific energy for electrochemical cells. An
opportunity exists for the development of systems that have the
capability of storing 5 to 10 times more energy per unit weight than the
Pb-PbO2 cell.
More specific barriers to meeting the goals include the high cost of
electrocatalysts and some porous electrodes; the prevention of corrosion
of active and passive cell components; instabilities of porous electrode
structures under long-term cycling; loss of electrocatalytic activity
with time and use; susceptibility of electrolytes to oxidation and/or
reduction by electrode reactants; inadequate conductivity of
electrolytes for high-performance applications; inadequate membranes and
separators (low chemical stability and conductivity); passivating film
formation on electrodes; and lack of advanced electrode and cell designs
for high-performance applications.
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TABLE 5-1 Performance Requirements for Batteries in Advanced Applications
Specific Specific
Battery Energy Power Efficiency Lifetime Cost
Application (Wh/kg) (W/kg) (%) Cycles (years) ($/k\\'h)
Autos and vans >70 >120
>60 >300 >3 <100
Stationary energy
storage n/a n/a >70 >2000 >10 <100
Portable power for
electronics >250* >2 n/a primary various 4000
Weapons (example) >100 >200 -- various various -
NOTE: Efficiency (%) = percentage of theoretical efficiency.
*The additional volumetric requirement of >0.6 Wh/cm3 is very important.
TABLE 5-2 Performance Requirements~for Fuel Cells in Advanced
Applications
Specific
Fuel Cell Power Efficiency Startup Cost
Application (W/kg) (%) Time Lifetime ($/kW)
Autos and vans 120 >30 <20 see
Stationary utility n/a >40 <1 hr
Weapons (example) >1000
>3 years <75
>10 years <1000
< 1 min < 1 hour
Space power >100 >40 5000 hours -
NOTE: Efficiency (%) = percentage of theoretical efficiency.
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47
A somewhat more general consideration of the battery and fuel-cell
field reveals a number of generic problems that are important in
numerous other electrochemical systems:
· The dimensional and morphological stability of porous electrode
structures under operating conditions
· Chemical and physical control of electrocrystallization of
metals and their solid discharge products
· Gas evolution at electrodes (H2 and/or O2 in aqueous systems)
· Electrocatalysis of O2 reduction and evolution
· Optimization of transport processes in porous electrode systems
(gases, ions, electrons, solvents)
· Electrocatalysis of the oxidation of logistic fuels
(hydrocarbons' reformer gas, methanol, coal)
Suppression of passive film formation
~ Advanced methods for the design and optimization of electrodes,
cells, and electrochemical systems
· Advanced methods for in situ study of electrochemical and
chemical reactions in porous electrodes and immobilized electrolytes
A plan for a more vigorous electrochemical R&D program (at a funding
level at 2 to 3 times the present value) would, for research, enhance
the funding and staff of existing programs of electrochemical research
and focus added effort on the generic problems discussed here. For
development, the plan would establish initiatives (described in
Chapter 4) for each of the systems undergoing development berg., Na-S,
Zn-Br2, Li-FeS2, H2-M2CO3-air, H2-ZrO2-air).
BIOMEDICAL SCIENCE AND HEALTH CARE
The origin of electric potentials in biological systems arises from
the existence of free ions, ionized molecular groups, or electrically
polarized biomolecules. In addition, electrical potentials accompany
charge transfer processes during the reaction of biologically active
systems. Although many processes that occur in biological systems lie
outside the scope of this report, and although advances in these areas
are likely to be made in a wide variety of disciplines, there are some
key areas where electrochemical phenomena play a significant role. For
example, the processes characteristic of living systems, such as active
transport and secretory processes, photosynthesis, sensory and energy
transduction, conduction and transmission of impulses, motility, and
reproduction, are all based on interactions between ions, poly-
electrolytes (proteins, DNA), or charged membranes containing enzymes
and ion-selective channels. The units of these biological structures
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are charged, and their interactions involve electrical forces. An
understanding' of life processes may thus be greatly aided by
collaboration with individuals who possess a thorough grounding in
electrochemical concepts and techniques. Such knowledge is also
indispensable for developing ways of utilizing information about
biological processes for industrial or medical applications (5~.
The five examples that follow are illustrative but not inclusive of all
areas where electrochemical phenomena represent an essential component.
Mechanism of Enzyme Catalysis
It is possible to carry out investigations of the electrochemical
properties of proteins and enzymes in biological oxidation-reduction
reactions in the native state. Highly significant is the fact that
there is sometimes direct exchange of electrons between the protein
molecule's active center and the electrode. The thermodynamics of the
redox centers have been evaluated electrochemically with the use of
indirect coulometric titration. The mechanisms of such electron
transfer reactions, however, are not always obvious. Primarily, the
role of the protein surface, and hence the pathway of electron transfer
from the electrode to the redox center, is not well understood.
Understanding of such phenomena will be quite valuable in resolving more
difficult questions on the mechanism of electron transfer between redox
centers when those centers are not directly accessible to an electrode.
Model studies are needed for dioxygen and dinitrogen metabolism,
cytochrome P-450, neuroactive substances, and redox chemistry of sulfur
and selenium.' The use of complementary methods such as surface-enhanced
Raman spectroscopy to probe interracial interactions or proteins on
electrodes would represent an important contribution. The technological
incentive for this work arises from the possibility of such electrodes
serving as energy converters or for highly specific electro-organic
synthesis.
Neuroscience
Proteins are major components in dendritic nerve membranes and may
exhibit electroactivity i.e., the characteristic of being switched
between two states of differing ionic conductivities. Such electro-
activity is interesting because the electricity of the nerve impulse,
the unitary basis of information encoding in neural systems, is
generated in the dendritic membrane, which is composed of electro-
chemically active proteins in a lipid bilayer. Thus' by interacting
with neuroscientists in the investigation of neural information coders),
electrochemists may make fundamental contributions to the molecular
elucidation of the human brain and the nervous systems of other major
animal species (6~.
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Technological applications emerging from such efforts include
energy-transduction and -amplifying devices, information encoding
devices for artificial intelligence systems, in vitro devices for
sensing oxygen and pharmacological agents with membrane-immobilized
proteins, and interface devices for organ or whole-body chemotherapy by
metered drug release.
These key scientific advances are needed in this area:
· Understanding of how complex ligands (e.g., messengers, drugs)
affect selectivity and sensitivity of ionic permeabilities of protein
membranes, films, or lamina
Improved film prototypes, such as conducting polymers involving
polypeptides, which might represent improved hosts for electroactive
protein insertion, as well as the characterization of a larger number
and variety of such proteins in order to improve knowledge of structure-
activity relationships, including the contribution of the protein to the
permeability-regulating capabilities of the laden film or membrane
· Better understanding of deterioration of ionic permeability,
usually associated with unwanted protein adsorption, in order to to
design synthetic systems that retain for practical periods their desired
capabilities
Cell Fusion
Cell-to-cell fusion can be achieved with the aid of electrical
stimulation (7-10~. Several techniques have been demonstrated in
which an electric field is applied for a short duration to point-
adhering (or agglutinated) cells, upon which fusion is immediately
induced. The fusion may be achieved by a single DC pulse, by a series
of pulses, or by gentle AC dielectrophoresis of a cell suspension.
Electrofusion has been successful in all types of cells tested to
date, including microbe and plant protoplasts, mammalian cells, and sea
urchin ova. One can (a) fuse unlike cells to create hybrid cells;
(b) fuse like cells to form larger entities such as giant cells 100 to
1000 times the volume of individual unit cells; and (c) help drive
external objects or chemical agents such as DNA into cells.
The mechanism of fusion is not understood. It is known that when
application of an external electric field causes the potential
difference across the separating membrane to reach a certain threshold
value, the membrane becomes reversibly transformed from the rest state
to a fusion-susceptible state, particularly in the contact zone between
adjacent cells. The membrane excitation in a broad sense is observed
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usually in milliseconds in animal cells and in seconds in plant cells.
In the fusion-susceptible state, the cell membrane or lipid bilayer
becomes more permeable to ions and macromolecules. ~ In addition, the
emergence of a protein-free domain occurs by lateral movement of
proteins away from the contact zone of the membranes between two cells.
The reversible electroporation of membranes at the contact zone leads
eventually to fusion.
The electrofusion technique is a significant new tool for research
and production of controlled systems in the life sciences. The study of
electric-field-induced membrane and cell phenomena on a molecular level
will contribute to fundamental understanding both of cell-to-cell fusion
and of membrane structure and function.
In Vivo Monitoring
In vivo measurements of chemical substances can be used to provide a
great deal of information concerning the regulation, metabolism, and
actions of various substances inside living organisms. Chemical sensors
based on electrochemical techniques are well suited for this applica-
tion, because they can be miniaturized so that minimal damage is caused
to the tissue to be probed. These electrochemical sensors can be used
to measure the distribution and concentration fluctuations of endogenous
substances or to study events in vivo such as drug partitioning between
different phases.
Ton-selective electrodes with tip diameters in the range of 0.5 to
lO,um have been developed for ions such as potassium, calcium, and
chloride, and these have been used to study the distribution of these
ions in both the extra- and intra-cellular fluid. These electrodes are
used in the potentiometric mode, and the specificity is established by
using a selective membrane that is only permeable to the ion of
interest. Voltammetric techniques have also been useful for in viva
measurements; the most widely used is the oxygen electrode, which
incorporates a polymer film that is only permeable to oxygen.
Electrode surfaces that have been properly modified with bioreactive
layers (enzyme, antibody, receptor) can provide access to the in vivo
investigation of biologically significant materials. Such devices offer
simplicity, low cost, miniaturization, automation and high sensitivity.
Key research areas include
Discovery of nolv,mer coatings that- maintain sensitivity. promote
O _ _ _ _ _ ~ _ _ _ _ _ _ _ _
selectivity, protect the electrode from the biological fluid, and
provide a biocompatible surface to the measured system
· Development of new and improved immobilized enzymes to increase
the scone of substances that can be detected by such techniques
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· Investigation of in viva environments with the use of very fast
electrochemical techniques for the elucidation of biologically
significant kinetic processes
Electrophoresis
Electrophoresis is defined as the transport of electrically charged
particles in liquid media under the influence of a DC electrical field.
In these techniques, ionic constituents separate either as a function of
their different rates of migration or by approaching zero mobility at
different locations in an equilibrium gradient (11~. One of the
most important applications of this spectrum of techniques is the
separation and analysis of complex mixtures of biological origin in
particular peptides, proteins, and nucleic acids. At present, two-
dimensional gel electrophoresis, combined with sophisticated computer
image analysis, is capable of resolving several thousand proteins among
the products of a given cell type (12~.
The most important applications of electrophoresis are in molecular
biology and medicine where, for example, the study of inherent
variabilities of serum proteins has produced a new branch of genetics,
and the discovery of hemoglobin variants in several anemias has
introduced the notion of molecular diseases. Electrophoresis has also
greatly facilitated sequencing of nucleic acids, the clinical diagnosis
of protein dyscrasias, the measurement of isoenzyme distribution, and
the classification of lipoproteinemias, among others.
In analytical applications the fluid is entrapped in a matrix, and
visualization of the electrophoresed one- and two-dimensional patterns
is done by staining, biological assays, or autoradiography, while data
analysis is typically performed by densitometry (11-13~.
Large-scale electrophoretic chambers (14-16) are currently being
investigated for fractionation and purification of pharmaceuticals and
other fermentation products on an industrial scale.
COATINGS AND FILMS
The need to modify the electrochemical properties of electrode-
solution interfaces has led to the development of a wide range of
coatings. The industry that coating technology supports has
multibillion-dollar annual sales and includes areas such as paints,
enamels, electrodeposits, and conductive polymers. As a result of
advances in the fields of surface modification, surface character-
ization, and adhesion, a revolution is occurring in coating technology.
In many cases it is now possible to design coatings having desired
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cation systems, the circuits are packaged in particularly expensive
ceramic hermetically sealed packages. Packaging and encapsulation now
constitutes 15 to 50 percent of the cost of microcircuits. If one adds
the expense of careful exclusion of ions in the processing steps (use of
deionized water, high-purity solvents, sodium-free reagents, etc.), the
cost of this ignorance of surface electrolytic processes in micro-
circuits is even higher.
Encapsulants of integrated circuits were originally introduced to
prevent mechanical damage and to slow down corrosion by reducing
transport of oxygen and water to the corroding metals. Today it is
recognized that encapsulants reduce corrosion by reacting with regions
on the hydrated SiO2 surface, thus slowing the lateral transport
of ions. Some encapsulants also act as ion traps.
It is reasonable to expect that, if methods for quantitative measure-
ment of the transport of ions in surface phases of semiconductors are
developed, the way will open to the exploration of chemical and physical
modification of these surface phases. The goal is to make these less
conductive solid electrolytes-i.e., surface phases in which ion transport
is reduced. Such modification is likely to reduce the cost of encap-
sulation and packaging and increase the reliability of microcircuits.
Reliability of Interconnecting Networks
Multilevel interconnecting networks consist of layers of metal
runners isolated from each other by a dielectric. At defined points,
runners in different planes are electrically contacted by metal
columns. The purpose of these three-dimensional networks is to carry
electrical signals at high speed. Therefore, the resistance and
capacitance of the interconnecting networks must be low. Low resistance
in a dense network of conductors implies that the runners must be made
of highly conductive metals such as copper. Low capacitance implies
that the layer of the dielectric must be thick and that its dielectric
constant must be low. Usually, the layers isolating the metal layers
are polymers like polyimides. Because oxygen diffuses to the polymer-
copper interface, the copper oxidizes. If complexing functions like
carboxylic acids are formed upon oxidation of the polymer or are
intrinsically present, they complex the copper cations, causing both
gradual dissolution of the metal and a change in the electrical
properties of the dielectric.
Because multilayer interconnecting networks are an important element
of advanced chips and parallel processors, it is essential that an
understanding of the corrosion processes that affect their reliability
be developed. Needed are methods to quantify metal corrosion and ion
transport in polymers and means to identify electrochemically reliable
metal-polymer systems.
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Electrochemistry of Highly Parallel Processors
The production of future generations of highly parallel processors
requires manufacturing processes of unprecedented stringency in yield
and precision. These processors will have dimensions of 10 to 100 cm2
and will consist of approximately 104 VLSI chips, with each chip
connected to every other chip by approximately 102 metal runners,
accommodated in a three-dimensional network. Their design requires. as
· , .~ e , · · ~
seen In the previous section, ~n-depth understanding of the interracial
electrochemistry between metals and dielectrics and of ion transport in
channels of diminishing size that connect metal runners in different
planes. Formation of the networks requires extreme control over the
plating process so that all columns have precisely identical lengths and
perfectly flat tops; nonidentical lengths or curved tops lead to defects
in the three-dimensional structure.
The most relevant areas of fundamental electrochemistry are modeling
of microcells and interracial corrosion.
Electrochemistry of Content-Addressable Memories
Beyonc} the evolution of von Neumann computers lies the beginning of
the science and technology of content-addressable memories now being
experienced. These approach more closely the way the human mind works.
They are more fault-tolerant and associative; i.e., they function with
imprecisely defined information and with imperfect circuit elements and
can relate information elements to each other. State-of-the-art
associative memories are based on variable-resistance network "opens"
and variable degrees of "shorts." The variable shorts can be generated
electrochemically both in polymers and in inorganic materials e.g., by
the reductive electrochemical diffusion of Na+ into WO3 films,
which produces conductive tungsten bronzes, or by the oxidative
diffusion of C1O4- into polyalkyl thiophene films, which
produces a conductive polymer. Such circuit elements have already been
made.
The most relevant areas of fundamental electrochemistry are
solid-state electrochemistry and the modeling of microcells.
Electrochemistry of Nerve-Electronics Interfaces
The electrochemistry of nerves has been the subject of several
decades of study. Ion transport across cell walls is a key element in
the functioning of nerve cells, and a network of nerves can be viewed as
a set of electrochemical, membrane-containing microcells that are
coupled by chemical messengers. Interfaces between nerves and
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microelectronic triggers that are crude by biological standards have
already been implemented and are in limited use in rehabilitation.
The modeling of coupled electrochemical microcelis, progress in
capacitive biocompatible microelectrodes, and the creation of precisely
tailored arrays of microelectrodes are particularly relevant to the
coupling of microelectronics and nerves.
SENSORS
Electrochemical sensors have demonstrated their potential to provide
sensitive, selective, reliable, robust, and inexpensive means for
solving otherwise intractable problems of chemical analysis (72~.
They have proved to be well suited for application to both gas phase and
liquid phase problems, including clinical chemistry and research in the
life sciences (73,74~. Some noteworthy devices include miniature
sensors for real-time monitoring of oxygen partial pressure in high-
temperature automobile exhausts, lightweight portable monitors for a
variety of toxic gaseous species (e.g., carbon monoxide, nitric oxide,
nitrogen dioxide, hydrogen sulfide), ion-selective electrodes for
measurement of electrolytes in clinical applications (sodium, potassium,
calcium, etc.), ultramicroelectrodes for in viva determination of
glucose and of biologically active species, detectors for liquid
chromatography of drugs used for neurological disorders and for
therapeutic drug monitoring, and potentiometric sensors for
quantification of low concentrations of electroactive species. With the
exception of potentiometric sensors, no consistent pattern of federal
support has existed.
Recent advances in microelectronic fabrication techniques, in
development of modified electrode surfaces and ion-selective membranes,
and in availability of new materials give promise for development of new
electrochemical sensors. For both gas and liquid sensors, the
possibility of much higher sensitivity exists. Lower detection limits
are possible for environmental, clinical, and general analysis
situations. Sensors developed to date are primarily based on classical
and relatively unsophisticated approaches. With newer methodologies and
device designs, one may anticipate at least a ten-fold improvement in
detection limits.
Among the methods that have considerable promise but that are yet to
be significantly exploited are pulse electrochemical techniques,
impedance methods, flow-injection analysis, the use of nonaqueous
solvents in the sensor, the combined use of chemometrics and multi-
electrode measurements for analysis of complex mixtures, the use of
ultramicroelectrodes in applications outside the clinical and biological
areas, and rapid deaeration of flow systems.
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Electrochemical sensors are based on selective interracial charge
generation and localized charge transport. Within the past decade,
major advances have been made in recognizing basic principles that unite
the wide variety of systems encountered in practice. From these
principles and the working out of charge, potential, and composition
Profiles. Prediction of the properties of materials for the design and
, , ~
. . · ~ . ~ ~ ~ ~ . ~ ~ ~ ~.
construction of new sensors has proceeded at an increasingly rapla rate.
Key scientific challenges include the design of new molecules and
substrates that possess the high transport selectivity required for new
and improved sensors. The discovery and molecular characterization of
new sensing elements will include surfaces modified with specific
electrocatalysts and/or enzymes, ion-specific membranes, fast ion-
conducting ceramics and glasses, conducting polymers, and semiconductor
materials. The use of surface analytical techniques to probe the
molecular details of the sensing mechanism of these materials will
contribute to improved sensitivity i.e., reduced interference by other
species. Closely related is the problem of sensor design for use in
very low concentrations of species. Theoretical characterization of
transport of sensed materials to and from the sensor interface must
advance significantly to design reliable and reproducible sensors and to
predict their responses in the transient and steady states. The
invention of new devices would be aided significantly by transposing the
principles of potential- and current-generating sensors to related
field-effect devices, by capitalizing on improved knowledge of
permselectivity in polymer films, and by exploring more deeply the
principles of charge cancellation reactions for immunological
applications.
Invention of new manufacturing methods based on the microelectronics
industry, coupled with new sensing materials and methods of detection,
would represent a significant advance. For example, new sensors based
on redundant arrays of microsensing devices may be key to low-cost
reliability, which is essential to many applications.
A significant barrier to developing improved sensors is the lack of
focus for support of fundamental studies and the inadequate marshalling
of multidisciplinary skills for development efforts. Much sensor
development now occurs in connection with health science needs, defense
needs, or the requirements of other mission-oriented agencies. Without
a focus of support, it is currently difficult to undertake fundamental,
systematic studies that would explore a new generation of sensing
techniques and materials. Sensor technology is multidisciplinary, both
in the assembly and characterization of the sensing element and in the
fitting of that element into the specific system in the field.
Manufacturers of instruments often do not have specialist teams with
adequate breadth to develop novel techniques into commercial devices.
As a consequence, there are missed opportunities in the conception of
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new methods as well as poor transfer to the marketplace of those
concepts that do arise.
In general there appear to be no generic problems that are inherent
to the development and fabrication of vastly improved electrochemical
sensors. The environment in which a sensor operates may generate
materials problems (such as in blood or at high temperature or
pressure), but these are not appreciably different from those existing
for other instruments and devices exposed to the same environment. It
is unlikely that more sophisticated sensors would give rise to intract-
able materials or manufacturing problems.
The present role of the federal government in support of sensor
science and technology is unfocused. There is no clearly evident
federal funding agency where a fundamental sensor proposal might attract
funding without being directly linked to a specific mission-oriented
problem. Improved federal sponsorship of fundamental investigations
aimed at developing principles of advanced sensors would play a major
role in promoting technological progress. The commercialization of new
and improved sensors by U.S. manufacturing firms represents a very
significant and strategic economic benefit.
Research areas that hold high promise for advancing technological
growth include
~ Enhancing sensor selectivity by discovery and molecular
characterization of new and improved sensing elements
· Invention of new fabrication methods, based on microdevice
technology, to improve reliability, reproducibility, and cost
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Representative terms from entire chapter:
fuel cell
